In microfluidics systems, it in often necessary to detect the presence or absence of a liquid or to detect the presence or absence of a gas at a predetermined location in such systems. For example, U.S. patent application Ser. No. 09/114,978, the entire disclosure of which is incorporated herein by reference, assigned to the assignee of this disclosure, discloses bubble valve-based pressure regulators that operate in response to pressure sensors that use a capacitative detector to detect the level of liquid in a capillary array. U.S. patent application Ser. No. 09/116,427, the entire disclosure of which is incorporated herein by reference, and also assigned to the assignee of this disclosure, discloses pressure regulators that include an active primary pressure regulator and a passive secondary pressure regulator. An array of capillaries is used as the secondary pressure regulator. The primary pressure regulator is controlled by a controller that operates in response to a capacitative pressure sensor coupled to the capillary array. Some embodiments of the capacitative pressure sensor detect the presence or absence of liquid in ones the capillaries constituting the capillary array. Other embodiments detect the liquid level in the capillary array.
In a United States patent application simultaneously filed with this disclosure and entitled Gas Extraction Device for Extracting Gas from a Microfluidics System, the entire disclosure of which is incorporated herein by reference, two of the inventors and others disclose a device for use in a microfluidics system that extracts from the system gas released from the liquid in the system. Some of the embodiments disclosed employ an active control system that operates in response to a detector that detects when a bubble of gas that accumulates at a location in the system has grown to a size that justifies extraction.
Conventional capacitative detectors for detecting the presence of a liquid typically include a pair of opposed conductive plates and a capacitance detector circuit that measures the capacitance between the plates. Interleaved conductive fingers located on one surface may be used instead of the opposed conductive plates, and the term plates will be understood to encompass such interleaved fingers. The capacitance detector circuit applies an alternating signal having an amplitude of several volts between the plates to measure the capacitance. When measured in this way, the capacitance between the conductive plates is proportional to the area of the plates and the dielectric constant of the medium separating the plates, and is inversely proportional to the distance between the plates. Since liquids generally have a greater dielectric constant than gases, the capacitance measured between the plates when the plates are in contact with a liquid is greater than that measured when the plates are in contact with a gas such as air. However, the change in capacitance is relatively small, and can be masked by the stray capacitances between the conductive plates and other elements of the microfluidics system. Consequently, it is often difficult to detect whether the plates are in contact with the liquid or not. This is especially true when a simple, low-cost capacitance detector circuit is used. The detection reliability can be increased by increasing the area of the plates, but restraints imposed by the small dimensions of the locations where the plates are positioned to detect the presence or absence of the liquid often prevent the area of the plates from being increased sufficiently to provide the sought-for detection reliability.
Thus, what is needed is a capacitative gas/liquid detector that can reliably and easily detect a change in capacitance caused by a liquid contacting a sensor. What is also needed is a capacitative gas/liquid detector that can easily be fabricated using the same micromachining techniques used to fabricate the other major elements of the microfluidics system.